Re: 48-bit sequence number arithmetic

[Eddie Kohler <kohler@cs.ucla.edu>]

Hi Gerrit,

Thanks for the useful work.  This is just for the record.

Your solution differs than the usual 32-bit subtraction solution used 
for TCP, but it has interesting properties of its own.  We now have 
pairs of numbers where "a != b && !before48(a, b) && !before48(b, a)"!

Not clear this is significantly better than having numbers where "a != b 
&& before48(a, b) && before48(b, a)".  As RFC 1982 says (and FYI this is 
explicitly referenced in RFC4340), "the problem case is left undefined, 
[and] implementations are free to return either result, or to flag an 
error".  This explicitly blesses the existing TCP implementation of SEQ_LEQ.

So you have made a perfectly reasonable choice, but it really isn't 
"more correct" than the other.  As a result, I wouldn't necessarily push 
too hard on the TCP folks; if I were them, I'd stay with the existing, 
de facto standard SEQ_LEQ.

Eddie


Gerrit Renker wrote:
> Hi Eddie,
> 
> sorry there is a bit of confusion here. For the record, please let's move the
> issue whether RFC 4340 is right or not out of the focus. If you say it is right,
> I will not argue with it; but there are useful and valid points that can be
> used to make existing algorithms better. 
> 
> And I think it is time well spent to think these issues through, in particular since
> the performance of CCID 3 for instance depends on the accuracy with which loss is 
> detected - therefore I don't think that it harms to strive for maximum precision in
> these matters.
> 
> 
> |  - Patches to clean up sequence number arithmetic are fine.
> |  - Your analysis of the 2^47 problem is not correct, however.  As RFC 1982 
> |  says, two 48-bit numbers which are 2^47 apart are *unordered*.  Think about 
> |  it: You see 0 and 2^47.  The distance between 0 and 2^47 is, IN EITHER 
> |  DIRECTION, exactly 2^47.  Neither can be declared before the other.
> Although RFC 1982 is about serial numbers as they are used in the Domain Name
> System, it is a very useful reference here. Your point is valid, and it is solved
> by the solution below. My point here is that for 2^(n-1) the result should really be
> 'undefined': with the current solution of subtraction, the result is not undefined,
> but ambiguous.
> 
> |  - This is exactly the same case as in 32-bit TCP sequence number comparisons. 
> You are right and therefore RFC 4340 is not `wrong'. It is strange that this way
> of comparing sequence numbers has survived for so long: as early as 4.4BSDLite
> (the SEQ_LEQ macros in Stevens vol II), until today's Linux IP stack.
> 
> 
> |  - Therefore I'd recommend staying with the simplest check you can find, which 
> |  may be the 64-bit trick recommended by RFC4340.
> I found a solution which is as easy to implement as that _and_ removes the ambiguity.
> It is, for two n-bit sequence numbers a and b, as follows:
> 	
>  	a `before' b <=>         1 <= b-a <= 2^(n-1) - 1
> 
> To contrast: the previous definition was:
> 
> 	a `before' b <=>   2^(n-1) <= a-b <= 2^n-1
> 
> and it suffers from the ambiguity problem when a-b = 2^(n-1). With the former solution,
> the ambiguity is removed: whenever the difference between a and b is 2^(n-1), the result
> is 2^(n-1) and thus neither a `before' b nor b `before' a: this is exactly what RFC 1982
> suggests.
> 
> And my suggestion is not even new: we say "it is 29 minutes /before/ xxx o'clock", but we
> don't say "it is /half/ before xxx o'clock". 
> 
> To summarise, the revised algorithm is:
> 	* store 48-bit numbers in leftmost fields of 64-bit numbers as per RFC 4340
> 	* a sequence number comparison based on the following pseudo-code:
>   		int before48(u64 a, u64 b) { return ((b << 16) - (a << 16)) > 0; }
> 	* this removes the ambiguity
> 	* same suggestion was made for 32-bit TCP sequence numbers to netdev@vger
> 
> 
> Gerrit